Hard disk drives are the most commonly employed data storage devices in personal computers, which are being continuously improved to meet the demand for higher performance. The amount of data to be stored on a hard disk drive is rapidly increasing due to more sophisticated software applications and larger data files. At the same time, personal computers are becoming smaller in size. For example, new types of portable computers known as notebooks and miniaturized desktop computers have been introduced to meet the demands for less space consumption in an office environment.
The reasons described above together with an additional demand for faster access to the data stored on the hard disk drive are forcing the manufacturers to reduce the size of hard disk drives.
A typical hard disk drive includes the following: a number of spinning disks stacked above each other on a spindle, a disk controller, a rotary actuator and an actuator retract circuit. All these elements are mounted in a chassis or housing and supplied with external cable connectors.
The spinning disks have a magnetic recording layer for data storage. The rotary actuator consists of a number of arms equipped with heads for reading and writing data in generally radial and concentric tracks in the recording layers of the individual disks. The actuator is usually driven by an attached voice coil motor(VCM). Flexible cables are connected with the actuator and the controller to transmit signals to and from the heads and to power the VCM. The disk controller is typically an electronic circuit that controls all functions of the hard disk drive.
During regular operation of the drive, the controllers control the actuator motions including the movements to and from a parking position, at which the actuator is placed when the drive is not under operation. However, if the power supply to the drive is shut off unexpectedly, the actuator may not be in the proper parking position. Since the controller requires external power to operate, it cannot park the actuator after unexpected power supply shut-off. An independent retract circuit parks the actuator in such cases.
Such a retract circuit has to be able to power and control the retraction or withdrawal of the actuator from the disk surface into a parking position within a critical time period during which the spinning disks slow down to a minimal rotational speed. The minimal rotational speed guarantees sufficient supporting airflow between the disk surface and an air bearing surface of the read and write heads to keep them at a minimum flying height. In case the supporting airflow should fall beneath a critical value, the heads are likely to crash and damage the disk surface. Moreover, if the heads were to come to rest on the smooth disk surface, they would adhere to the disks through a process known as stiction.
There are two main methods for parking heads during power-off of the disk drive. The first is known as contact start-stop (CSS). In this method, the heads are moved to a special central location of the disks, the so-called landing zone. During the down spinning period of the disks, the supporting airflow decays and the heads land on the landing zone. To prevent stiction between the heads and disks, this landing zone is roughened or textured.
One of the main shortcomings of CSS is wear between the heads and the disks, which is caused by a sliding contact during the landing process. Another problem is that the flying height of heads in modern disk drives is becoming lower than the minimum required surface roughness, such that the heads tend to contact the textured disk surface, while the disks are still rotating at full speed. As a result, the interface is subject to excessive wear.
External imposed mechanical shock causes the heads, parked on the central landing zone to vibrate with the danger of vertically impacting the disk surfaces, an effect commonly known as head slap. Head slap can damage the heads and disks, and can generate particles that could cause the heads to crash.
Another method for parking heads during power-off is to move them to a parking ramp, located at either the inner or outer diameter of the disks. Each head of an actuator is mounted on a suspension. A tab or sliding element is provided at the end of the suspension. The tab is pushed onto the wedge shaped ramp. When the tabs reach the predetermined parking position on the ramp, the actuator is held in place with either a mechanical or an inertial latch. A mechanical latch locks the actuator in place, while an inertial latch engages a mechanism against the actuator when the drive is exposed to mechanical shock or acceleration. This ramp-parking process overcomes some of the shortcomings of CSS parking described above. Since the heads are not sitting on the disk during power-off, they are less prone to damage from external shocks. This is especially important in portable disk drives.
The electrical energy necessary for the retract movement to a parking position and the internal energy consumed by the actuator retract circuit is typically generated by a back electromotive energy generated from the kinetic energy stored in the rotating disk stack. The kinetic energy is thereby converted into a back electromotive voltage (BEMV) by utilizing the disk motor as a generator. The rectified BEMV is electrically connected across the voice coil motor VCM, which generates a torque on the actuator in the desired direction into the parking position. An example of such a retract circuit is described in U.S. Pat. No. 5,486,957.
As disk drives are made increasingly smaller for portable applications, several design parameters become limiting to the use of spindle motor BEMV for the retract event. The primary demand to save battery power reduces the size and mass of all moving or accelerated parts with the following results: First, the disk drives employ fewer disks of smaller diameter and the disk motors have lower torque constants and a relatively high internal resistance. These parameters reduce the magnitude of the back electromotive voltage that can be obtained from the spinning motor, as well as the total energy that can be extracted to park the actuator. Secondly, the VCM also has a low torque constant and an increased internal resistance due to a reduced wire diameter of the windings. Since the bias force from the actuator cables is not reduced proportionally with the torque constant of the VCM, the minimal required retract energy is higher relative to that in conventional disk drives.
Some parameters become more advantageous, such as the lower moment of inertia of the shorter actuator, and a reduced number of heads with their tabs that have to be moved onto a parking ramp.
Still, the combination of all parameters impose an increase of the demanded retract energy relative to the electric energy that can be extracted from the kinetic energy of the down spinning disk stack.
To overcome these problems in small drives, an alternate approach is to power the retract circuit with energy stored in a capacitor. A combination of both types of energy sources may also be used.
U.S. Pat. No. 4,786,995 to Stupek teaches a retract system for use with a stepper-motor actuator. Energy for the stepper motor is derived from the back electromotive energy generated from the down-spinning disk stack assisted by a charged capacitor. Highly complicated electronics are used to supply the energy in a number of pulses with a predetermined rate that defines the velocity and the rotational angle of the stepper motor. The stepper motor imposes its controlled rotation over a geared transmission system onto the actuator. The retract system is sophisticated and relies heavily on resistive components, which increase internal energy consumption. Even in hard disk drives equipped with large diameter disks and a relatively large amount of available kinetic energy additional capacitors have to be used to power the electronics and the stepper motor for the retraction and parking. The retract system is not feasible for small hard disk drives that use voice coils.
U.S. Pat. No. 5,325,030 to Yamamura et al. describes the general use of capacitors as possible energy source for a voice coil actuator retract. To initiate the retraction the capacitor is simply connected to the voice coil actuator to provide a single energy pulse. The actuator is accelerated and moves away from the disk surface. A shortcoming of single-pulse type capacitor retract systems is that the actuator may not properly enter and stay in the parking position under all circumstances. At the moment of power-off, the velocity and position of the actuator can vary over the operational range of positions and velocities during the regular usage of the drive. The torque needed to reach the parking position varies widely depending on these initial conditions. With a single pulse retract, without any adjustment for initial conditions, a predetermined torque pulse is applied to the actuator regardless of initial conditions. In some situations the torque pulse may be too large, such that the actuator bounces out of the parking position, which results in drive failure.
U.S. Pat. No. 5,495,156 to Wilson et al. discloses an actuator retraction circuit that uses the back electromotive energy generated from a down spinning disk stack to move the actuator into a parking position. The velocity of the actuator in or against retraction direction at the beginning of the retraction event generates an electromotive voltage, which is utilized to adjust the amount of energy imposed in an initial energy pulse onto an actuator motor. The invention sufficiently compensates the velocity differences of the actuator at the beginning of the retraction event to park the actuator with a predetermined end velocity. Because of the sophistication of the circuit with its reliance on resistors its internal energy consumption is too high to utilize it in very small disk drives. The disclosed invention is also only designed to impose most of its energy in an initial impulse leaving no energy reserves for the moment when the actuator reaches the ramp to ensure the proper retention in the predetermined parking position.
U.S. Pat. No. 5,615,064 to Blank et al. and U.S. Pat. No. 5,663,846 to Masuoka and Toru describe actuator control systems that use a back electromotive voltage from the actuator motor as a control signal during regular operation of the hard disk drive. The hard disk drive has an external energy supply. The control signal is processed together with other derived parameters within the hard disk controller for a regular operational retraction. The back electromotive voltage is changed into a control signal with a relative high energetic expenditure. The disclosed inventions are thus not useful for a power-off retract circuit with a very limited energy supply derived from capacitors.
A further shortcoming of capacitive retract systems is that they require physically large and expensive capacitors to store energy to power the retract. In a very small drive, such as a 1-inch drive in a CompactFlash form factor, it is challenging to budget enough space for a large capacitor. Since single-pulse capacitive retract circuits are inefficient in their conversion of stored energy into mechanical energy of the rotary actuator, the capacitors have to be larger than necessary.
Therefore, there exists a need for an actuator retract circuit with a low internal energy consumption and a highly efficient distribution of the available energy to retract an actuator from its operational position over a disk into a fixed parking position. The current invention addresses this need.